A spark plug for use in internal-combustion engines, such as the engine of an automobile includes, for example, a center electrode, an insulator provided outside thereof, a cylindrical metal shell disposed outside of said insulator and a ground electrode whose rear portion is joined to a front-end face of said metal shell. The ground electrode assumes a generally rectangular form in the cross-section and is disposed so that a front-end portion inner side face thereof faces the front-end face of the center electrode. Thus, a spark discharge gap is formed between the front-end portion of the center electrode and the front-end portion of the ground electrode.
A screw portion (not illustrated) is formed on an outer circumferential face of the metal shell. A spark plug is mounted on an engine in such a manner that the screw portion of the metal shell screws into a female thread of a plug opening formed in the engine cylinder head. When the spark plug is mounted such that an air-fuel mixture is exposed to a back face of the ground electrode, there is a possibility that the inflow of the air-fuel mixture to the spark discharge gap may be disturbed by the ground electrode. As a result, the ignitability of the spark plug is unlikely to be stable.
On the other hand, the conventional art discloses a spark plug having two or more ground electrodes therein in which each ground electrode assumes a columnar form with a generally circular-shape in the cross-section (e.g. Japanese Patent Application Laid-Open (kokai) No. H11-121142). Thus, with forming the ground electrode into the generally circular-shape in the cross-section, the air-fuel mixture is unlikely to be exfoliated from the ground electrode and easily reaches the spark discharge gap by flowing to the inner side of the ground electrode, even in a case where the air-fuel mixture is exposed to the back face of the ground electrode.
However, since a ground electrode is joined to a front-end face of a metal shell, the cross-sectional area of the circular-shaped ground electrode has to be smaller than that of the rectangular-shaped ground electrode. As a result, so-called heat sinking ability (heat dispersion) of the ground electrode deteriorates, and the temperature thereof tends to increase at the time of high speed driving or the like. Consequently, the ground electrode suffers considerable erosion which leads to a poor durability thereof.
Therefore, it is thought that the ground electrode is formed with a two-layer structure comprised of an outer layer comprised of a nickel alloy, which is excellent in oxidation resistance, and an inner layer comprised of a metal having a better thermal conductivity (e.g., copper-system metal) than that of the outer layer. However, when the materials constituting the outer layer and the inner layer differ, a deformation (i.e., “spring back”) of the ground electrode tends to occur due to a difference in the coefficient of thermal expansion between the outer layer and the inner layer, thereby possibly influencing the spark discharge gap. Specifically, a defect of the spark plug caused by such deformation is likely to occur when an outer diameter of the ground electrode is relatively small (e.g., 2 mm or less).